Lateral double-diffused metal-oxide-semiconductor (LDMOS) is a commonly used power device. The performance of an LDMOS is generally assessed by its breakdown voltage and on-resistance, and it is desired to manufacture an LDMOS device with a high breakdown voltage and a low on-resistance. However, it is very difficult to achieve a high breakdown voltage and a low on-resistance at the same time, as the increase in the breakdown voltage will usually lead to an increase of the on-resistance, while the decrease in the breakdown voltage will usually lead to a reduction of the on-resistance. Compared with the structure of a MOS device, an LDMOS device has added a diffused well between the gate and the drain. Such a structure may greatly increase the breakdown voltage, but may also result in a relatively high on-resistance. In order to reduce the on-resistance, superjunction LDMOS devices have been proposed.
FIG. 1 is a schematic diagram illustrating the structure of an existing superjunction LDMOS device. As shown in FIG. 1, the superjunction LDMOS device has a drain drift region, which is composed of a diffused well 10 and a superjunction structure 11. The superjunction structure 11 is formed by alternatively arranged P-type regions and N-type regions. When a voltage is applied between a drain and a source of the superjunction LDMOS device, an electrical current will flow through the superjunction structure 11. As the P-type and N-type regions have the same shape and the electrical charges in the P-type and N-type regions are balanced, the on-resistance of the device can be greatly reduced.
With the increase of the breakdown voltage, the depth to width ratio of a superjunction structure 11 is required to be correspondingly increased. In ultra-high voltage devices, special processes, such as the process of forming stacked epitaxial layers, are needed to achieve a desired depth to width ratio of the superjunction structure 11, which may lead to asymmetries between the P-type and N-type regions and an increased manufacturing cost due to the extremely complicated processes.